Online Computer Terms Dictionary - P

primary key

<database> A unique identifier, often an integer, that labels a certain
row in a table of a relational database.

When this value occurs in other tables as a reference to a particular row in the
first table it is called a "foreign key".

Some RDBMSes can generate a new unique identifier each time a new row is
inserted, others merely allow a column to be constrained to contain unique
values.

A table may have multiple candidate keys, from which the primary key is chosen.
The primary key should be an arbitrary value, such as an autoincrementing
integer. This avoids dependence on uniqueness, permanence and format of existing
columns with real-world meaning (e.g. a person's name) or other external
identifier (e.g. social security number).

There should be enough possible primary key values to cater for the current and
expected number of rows, bearing in mind that a wider column will generally be
slower to process.